MITICIDE BIOASSAYS WITH SPIDER-MITES (ACARI, TETRANYCHIDAE) - EFFECT OF TEST METHOD, EXPOSURE PERIOD AND MORTALITY CRITERION ON THE PRECISION OF RESPONSE ESTIMATES

Six different bioassay methods were evaluated using propargite (Omite 30% wettable powder (WP) and fenbutatin oxide (Torque 50% (WP) and 55% suspension concentrate (SC)) with twospotted spider mite, Tetranychus urticae Koch (TSM) and European red mite, Panonychus ulmi Koch (ERM) to document their utility and precision for estimating median lethal c oncentrations (LC). For each method, two post-treatment exposure perio ds and mortality criteria were used. Post-treatment exposure period an d mortality criterion had a significant influence on the precision of LC50 estimates for all tested miticides with all bioassays methods. Tw enty four hour (h) post-treatment exposure was found to be the most su itable for the slide dip and Petri dish methods while 48h was the most appropriate for leaf disc methods. Scoring moribund mites as dead was the most satisfactory criterion for ensuring that biossays were as si mple and precise as possible. The Petri dish residue-Potter tower meth od (PDR-PT) estimated the responses of TSM and EPM to propargite with high precision. The same method was not as precise for fenbutatin oxid e formulations. Because significant mite run-off occurred with the lea f disc methods, their precision was not fully established. The slide d ip method gave less precise estimates of LC50 values for propargite (W P) and fenbutatin oxide (WP), while the same method gave more precise LC50 estimates for fenbutatin oxide (SC) than the PDR-PT method. The t oxicity of candidate miticides was found to be method-and species-depe ndent.